Since zinc (Zn) plating, suppressing the corrosion of iron (Fe) using cathodic protection, has excellent anti-corrosion performance and economic efficiency, Zn plating has commonly been used in manufacturing a steel material having high corrosion resistance. In detail, in the case of hot dip galvanized steel materials forming a plating layer in such a manner that a steel material is immersed in molten Zn, the manufacturing process thereof is simpler, and the prices of products are cheaper than an electrogalvanized steel material. Thus, demand for hot dip galvanized steel materials has increased in the automobile industry, the household appliance industry, the building materials industry, and other industries.
Hot dip galvanized steel materials plated with Zn may have the characteristics of sacrificial corrosion protection in which, when being exposed to a corrosion environment, Zn having an oxidation reduction potential lower than that of Fe is first corroded, so that the corrosion of steel materials is prevented. In addition, Zn in a plating layer may be oxidized to generate minute corrosion products on the surface of steel materials and to block steel materials from an oxidizing atmosphere, thereby improving corrosion resistance.
However, due to increased air pollution caused by industrial development, an increase of corrosive environments and strict regulations regarding resource conservation and energy savings, there is a growing need for the development of steel materials having better corrosion resistance than Zn plated steel materials of the related art.
To this end, a large amount of research into technology to manufacture Zn alloy plated steel materials having improved corrosion resistance through adding elements, such as aluminum (Al) and magnesium (Mg) to a galvanizing bath, has been conducted. A large amount of research into a technology to manufacture Zn—Al—Mg-based Zn alloy plated steel materials, as a typical Zn alloy plated material, in which Mg is added to Zn—Al plated materials has also been conducted.
However, such Zn—Al—Mg-based Zn alloy plated steel materials have weaknesses as below.
First, when Zn—Al—Mg-based Zn alloy plated steel materials are welded, cracks caused by liquid metal embrittlement (LME) may easily occur, thereby degrading weldability thereof. In other words, in a case in which Zn alloy plated steel materials described above are welded, Zn—Al—Mg-based intermetallic compounds having a relatively low melting point are dissolved and penetrate between grain boundaries of the base steel.
Second, Zn—Al—Mg-based Zn alloy plated steel materials have a low level of deformed-part corrosion resistance. In other words, such Zn alloy plated steel materials may include a large amount of Zn—Al—Mg-based intermetallic compounds generated by thermodynamic interactions of Zn, Al, and Mg in a plating layer. Since such intermetallic compounds have a relatively high degree of hardness, cracks may be generated in a plating layer during a bending process, thereby degrading deformed-part corrosion resistance.